1. Field of the Invention
This invention relates to a shipping container for nuclear fuel components and, in particular, to such a container for unirradiated nuclear fuel assemblies and nuclear fuel rods.
2. Related Art
In the shipping and storage of unirradiated nuclear fuel elements and assemblies which contain large quantities and/or enrichments of fissile material, U235, it is necessary to assure that criticality is avoided during normal use, as well as under potential accident conditions. For example, nuclear reactor fuel shipping containers are licensed by the Nuclear Regulatory Commission (NRC) to ship specific maximum fuel enrichments; i.e., weights and weight-percent U235, for each fuel assembly design. In order for a new shipping container design to receive licensing approval, it must be demonstrated to the satisfaction of the NRC that the new container design will meet the requirements of the NRC rules and regulations, including those defined in 10 CFR § 71. These requirements define the maximum credible accident (MCA) that the shipping container and its internal support structures must endure in order to maintain the sub-criticality of the fuel assembly housed therein.
U.S. Pat. No. 4,780,268, which is assigned to the assignee of the present invention, discloses a shipping container for transporting two conventional nuclear fuel assemblies having a square top nozzle, a square array of fuel rods and a square bottom nozzle. The container includes a support frame having a vertically extending section between the two fuel assemblies which sit side by side. Each fuel assembly is clamped to the support frame by clamping frames which each have two pressure pads. This entire assembly is connected to the container by a shock mounting frame and a plurality of shock mountings. Sealed within the vertical section are at least two neutron absorber elements. A layer of rubber cork cushioning material separates the support frame and the vertical section from the fuel assemblies.
The top nozzle of each of the conventional fuel assemblies is held along the longitudinal axis thereof by jack posts with pressure pads that are tightened down to the square top nozzle at four places. The bottom nozzle of some of these conventional fuel assemblies has a chamfered end. These fuel assemblies are held along the longitudinal access thereof by a bottom nozzle spacer which holds the chamfered end of the bottom nozzle.
These, and other shipping containers, e.g., RCC-4, for generally square cross-sectional geometry pressurized water reactor (PWR) fuel assemblies used by the assignee of the present invention, are described in Certificate of Compliance Number 5454, U.S. Nuclear Regulatory Commission, Division of Fuel Cycle and Material Safety, Office of the Nuclear Material Safety and Safeguards, Washington, D.C. 20555.
U.S. Pat. No. 5,490,186, assigned to the assignee of the present invention, describes a completely different nuclear fuel shipping container designed for hexagonal fuel, and more particularly, for a fuel assembly design for a Soviet-style VVER reactor. Still, other shipping container configurations are required for boiling water reactor fuel.
There is a need, therefore, for an improved shipping container for a nuclear fuel assembly that can be employed interchangeably with a number of nuclear reactor fuel assembly designs.
There is a further need for such a fuel assembly shipping container that can accommodate a single assembly in a lightweight, durable and licensable design.
These and other needs have been partially resolved by U.S. Pat. No. 6,683,931, issued Jan. 27, 2004 and assigned to the assignee of the instant invention. The shipping container described in this latter patent includes an elongated inner tubular liner having an axial dimension at least as long as a fuel assembly. The liner is preferably split in half along its axial dimension so that it can be separated like a clamshell for placement of the two halves of the liner around the fuel assembly. The external circumference of the liner is designed to be closely received within the interior of an overpack formed from an elongated tubular container having an axial dimension at least as long as the liner. Preferably, the walls of the tubular container are constructed from relatively thin shells of stainless steel and the liner is coaxially positioned within the tubular container with close-cell polyurethane disposed in between. Desirably, the inner shell includes boron impregnated stainless steel. The tubular liner enclosing the fuel assembly is slidably mounted within the overpack and the overpack is sealed at each end with end caps. The overpack preferably includes circumferential ribs that extend around the circumference of the tubular container at spaced axial locations that enhance the circumferential rigidity of the overpack and form an attachment point for peripheral shock-absorbing members. An elongated frame, preferably of a birdcage design, is sized to receive the overpack within the external frame in spaced relationship with the frame. The frame is formed from axially spaced circumferential straps that are connected to circumferentially-spaced, axially-oriented support ribs that fixedly connect the straps to form the frame design. A plurality of shock absorbers are connected between certain of the straps and at least two of the circumferential ribs extending around the overpack, to isolate the tubular container from a substantial amount of any impact energy experienced by the frame, should the frame be impacted.
Although the shipping container described in the aforementioned '931 patent is a substantial improvement in that it can accommodate different fuel assembly designs through the use of complementary liners while employing the same overpack and birdcage frame, that improvement has been taken one step further by U.S. Pat. No. 6,748,042, assigned to the assignee of the instant invention. The '042 patent describes a transport system that provides a liner and overpack system that will achieve the same objectives as the '931 patent while further improving the protective characteristics of the transport system and the ease of loading and unloading the nuclear fuel components transported therein. The shipping container includes an elongated tubular container, shell or liner designed to receive and support a nuclear fuel product such as a fuel assembly therein. The interior of the tubular liner preferably conforms to the external envelope of the fuel assembly. The exterior of the tubular container has at least two substantially abutting flat walls which extend axially. In the preferred embodiment, the cross-section of the tubular member is rectangular or hexagonal to match the outer envelope of the fuel assembly and three of the corner seams are hinged so that removal of all the kingpins along a seam will enable two of the sidewalls to swing open and provide access to the interior of the tubular container. The tubular container or liner is designed to seat within an overpack for transport. The overpack is a tubular package having an axial dimension and cross-section larger than the tubular liner. The overpack is split into a plurality of circumferential sections (for example, two sections, a lower support section and an upper cover, or three sections, a lower support section and two upper cover sections) that are respectively hinged to either circumferential side of the lower support section and joined together when the overpack is closed. The lower support section includes an internal central V-shaped groove that extends substantially over the axial length of the overpack a distance at least equal to the axial length of the tubular liner. Shock mounts extend from both radial walls of the V-shaped groove to an elevation that will support the tubular liner in spaced relationship to the groove. The axial location, number, size and type of shock mount employed is changeable to accommodate different loadings. The tubular liner is seated on the shock mounts, preferably with a corner of the liner aligned above the bottom of the V-shaped groove. The top cover section (sections) of the overpack has a complementary inverted V-shaped channel that is sized to accommodate the remainder of the tubular liner with some nominal clearance approximately equal to the spacing between the lower corner of the tubular liner and the bottom of the V-shaped groove. The ends of the overpack are capped and the overpack sections are latched.
Though the transport system of the '042 patent provides a substantial improvement in the protective characteristics and ease of loading and unloading of the nuclear fuel components being transported, further improvement in the ease of loading and unloading the liner is desired.